Towards optimal regimens for the UVB phototherapy of psoriasis: A mathematical model

A mathematical model is described that predicts the response of psoriasis to a treatment course of UVB irradiation. The basis of the model is that UVB acts by a direct effect on keratinocytes and that cell cycle arrest is the major mode of action in the phototherapeutic response in psoriasis. Although it is unlikely that UVB causes resolution of psoriatic plaques through a single mechanism, the current model has been based on epidermal cell cycle arrest and entry into the terminal differentiation compartment because this is likely to be a significant rate-limiting factor in determining response to treatment. The model has been validated against results obtained from published clinical studies on narrowband (TL-01) UVB phototherapy for psoriasis. The principal outcomes of the model are that for a given erythemal response, the number of exposures required for clearance is almost independent of the frequency with which patients attend for treatment and that the higher the exposure dose per treatment, the more rapidly will clearance result. The model has been used to suggest optimal regimens for the treatment of outpatients and inpatients.